Rownum In SQL

What is the ROWNUM clause in SQL, and how is it used?

ROWNUM in SQL is a pseudo-column that assigns a unique row number to each row in a result set, ordered by the query's default order. It's crucial for retrieving specific rows based on their position in the result set. It's often used for pagination and selecting top N rows.

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Description

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ROWNUM is a pseudo-column, not a regular column, meaning it's not stored in the table. It's assigned dynamically by the database system during query execution. The order of the rows in the result set is determined by the query's ORDER BY clause (if present). If no ORDER BY clause is specified, the order is arbitrary. Crucially, ROWNUM is assigned *before* any filtering or aggregation occurs. This means that if you filter the results, the row numbers will still reflect the original order. ROWNUM is particularly useful for tasks like retrieving the top 10 performers, or for implementing pagination in applications. It's important to note that ROWNUM is not guaranteed to be unique across multiple queries on the same table. Each query gets its own independent numbering.

Why Rownum In SQL is important

ROWNUM is a powerful tool for controlling the output of SQL queries. It allows developers to extract specific rows from a result set based on their position, which is essential for tasks like pagination, selecting top N records, and implementing complex data retrieval logic.

Rownum In SQL Example Usage


SELECT employee_id, employee_name, salary
FROM employees
WHERE department_id = 10
ORDER BY salary DESC
FETCH FIRST 5 ROWS ONLY;
--Alternative using ROWNUM
SELECT employee_id, employee_name, salary
FROM (
    SELECT employee_id, employee_name, salary, ROW_NUMBER() OVER (ORDER BY salary DESC) as rn
    FROM employees
    WHERE department_id = 10
) ranked_employees
WHERE rn <= 5;

Rownum In SQL Syntax

Common Mistakes

Forgetting to use ORDER BY with ROWNUM, leading to unpredictable results.
Misunderstanding that ROWNUM is a regular column, leading to errors in queries.
Trying to use ROWNUM with aggregate functions like SUM or AVG, which will not work as expected.
Assuming ROWNUM is globally unique across multiple queries.

Frequently Asked Questions (FAQs)

Why can a later WHERE clause make ROWNUM look confusing?

Oracle assigns ROWNUM immediately after it pulls the raw rows and before it evaluates your WHERE, GROUP BY, or HAVING clauses. When you subsequently filter the result set, the surviving rows keep the numbers they originally received, so the sequence still starts at 1 and preserves the original order even though some rows were removed. This behavior often surprises developers who expect ROWNUM to be recalculated after filtering.

What is the safest pattern for returning the top-N rows with ROWNUM?

Always perform the ORDER BY in a sub-query, then apply ROWNUM in the outer query:SELECT *FROM (SELECT * FROM employees ORDER BY salary DESC)WHERE ROWNUM <= 10;The inner query sorts the data first; the outer query numbers that pre-sorted data and limits it. This guarantees that the “top 10” really reflects the intended sort column rather than the database’s default retrieval order.

How can Galaxy help me build pagination queries that use ROWNUM?

Galaxy’s context-aware AI copilot can autogenerate the nested queries typically required for Oracle pagination (e.g., using ROWNUM > :offset and ROWNUM <= :offset + :limit). The editor lets you parameterize :offset and :limit values, preview the results, and share the finalized SQL with teammates in a Collection—so your team reuses one vetted pattern instead of copying partial snippets in Slack.